CPAN RT

According to the modern rules governing the Roman Numbers,
a numerical symbol cannot appear more than three times in a contiguous row.
Also,
the modern rules have forgotten the symbols for 5000 and more,
so the highest symbol is M for 1000.
From that,
you can conclude that the highest Roman number compatible with modern rules is MMMCMXCIX,
that is,
3999.
See http://datetime.mongueurs.net/divers/nombres-romains/Roman_Numbers.html.